wakame monad maybe http://connpass.com/event/1152/

MyMaybe.scala

abstract class MyMaybe[T] {
  def flatMap[U](f:T=>MyMaybe[U]):MyMaybe[U]
  def map[U](f:T=>U):MyMaybe[U]
}
case class MyNothing[T]() extends MyMaybe[T] {
  def flatMap[U](f:T=>MyMaybe[U]):MyMaybe[U] = MyNothing()
  def map[U](f:T=>U):MyMaybe[U] = MyNothing()
}
case class MyJust[T](elm:T) extends MyMaybe[T] {
  def flatMap[U](f:T=>MyMaybe[U]):MyMaybe[U] = f(elm)
  def map[U](f:T=>U):MyMaybe[U] = MyJust(f(elm))
}

val db = Map("x"->12, "y"->3, "z"->20)

def myget(db:Map[String,Int], k:String):MyMaybe[Int] = db.get(k) match {
  case Some(v) => MyJust(v)
  case None => MyNothing()
}

val c = for (a <- myget(db, "x");
             b <- myget(db, "y")) yield (a + b)
val f = for (d <- myget(db, "z");
             e <- myget(db, "w")) yield (d + e)